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Handle parsing errors

Occasionally the LLM cannot determine what step to take because its outputs are not correctly formatted to be handled by the output parser. In this case, by default the agent errors. But you can easily control this functionality with handle_parsing_errors! Let’s explore how.

Setup​

from langchain import hub
from langchain.agents import AgentExecutor, create_react_agent
from langchain_community.llms import OpenAI
from langchain_community.tools import WikipediaQueryRun
from langchain_community.utilities import WikipediaAPIWrapper

api_wrapper = WikipediaAPIWrapper(top_k_results=1, doc_content_chars_max=100)
tool = WikipediaQueryRun(api_wrapper=api_wrapper)
tools = [tool]

# Get the prompt to use - you can modify this!
prompt = hub.pull("hwchase17/react")

llm = OpenAI(temperature=0)

agent = create_react_agent(llm, tools, prompt)

Error​

In this scenario, the agent will error because it fails to output an Action string (which we’ve tricked it into doing with a malicious input

agent_executor = AgentExecutor(agent=agent, tools=tools, verbose=True)
agent_executor.invoke(
    {"input": "What is Leo DiCaprio's middle name?\n\nAction: Wikipedia"}
)


> Entering new AgentExecutor chain...
ValueError: An output parsing error occurred. In order to pass this error back to the agent and have it try again, pass `handle_parsing_errors=True` to the AgentExecutor. This is the error: Could not parse LLM output: ` I should search for "Leo DiCaprio" on Wikipedia
Action Input: Leo DiCaprio`

Default error handling​

Handle errors with Invalid or incomplete response:

agent_executor = AgentExecutor(
    agent=agent, tools=tools, verbose=True, handle_parsing_errors=True
)
agent_executor.invoke(
    {"input": "What is Leo DiCaprio's middle name?\n\nAction: Wikipedia"}
)


> Entering new AgentExecutor chain...
 I should search for "Leo DiCaprio" on Wikipedia
Action Input: Leo DiCaprioInvalid Format: Missing 'Action:' after 'Thought:I should search for "Leonardo DiCaprio" on Wikipedia
Action: Wikipedia
Action Input: Leonardo DiCaprioPage: Leonardo DiCaprio
Summary: Leonardo Wilhelm DiCaprio (; Italian: [diˈkaːprjo]; born November 1I now know the final answer
Final Answer: Leonardo Wilhelm DiCaprio

> Finished chain.
{'input': "What is Leo DiCaprio's middle name?\n\nAction: Wikipedia",
 'output': 'Leonardo Wilhelm DiCaprio'}

Custom error message​

You can easily customize the message to use when there are parsing errors.

agent_executor = AgentExecutor(
    agent=agent,
    tools=tools,
    verbose=True,
    handle_parsing_errors="Check your output and make sure it conforms!",
)
agent_executor.invoke(
    {"input": "What is Leo DiCaprio's middle name?\n\nAction: Wikipedia"}
)


> Entering new AgentExecutor chain...
Could not parse LLM output: ` I should search for "Leo DiCaprio" on Wikipedia
Action Input: Leo DiCaprio`Check your output and make sure it conforms!Could not parse LLM output: `I should search for "Leo DiCaprio" on Wikipedia
Action Input: Leo DiCaprio`Check your output and make sure it conforms!Could not parse LLM output: ` I should search for "Leonardo DiCaprio" on Wikipedia
Action Input: Leonardo DiCaprio`Check your output and make sure it conforms!Could not parse LLM output: ` I should search for "Leonardo DiCaprio" on Wikipedia
Action Input: Leonardo DiCaprio`Check your output and make sure it conforms!I now know the final answer
Final Answer: Leonardo

> Finished chain.
{'input': "What is Leo DiCaprio's middle name?\n\nAction: Wikipedia",
 'output': 'Leonardo'}

Custom Error Function​

You can also customize the error to be a function that takes the error in and outputs a string.

def _handle_error(error) -> str:
    return str(error)[:50]


agent_executor = AgentExecutor(
    agent=agent,
    tools=tools,
    verbose=True,
    handle_parsing_errors=_handle_error,
)
agent_executor.invoke(
    {"input": "What is Leo DiCaprio's middle name?\n\nAction: Wikipedia"}
)


> Entering new AgentExecutor chain...
Could not parse LLM output: ` I should search for "Leo DiCaprio" on Wikipedia
Action Input: Leo DiCaprio`Could not parse LLM output: ` I should search for I should look for a section on his personal life
Action: Wikipedia
Action Input: Personal lifePage: Personal life
Summary: Personal life is the course or state of an individual's life, especiallI should look for a section on his early life
Action: Wikipedia
Action Input: Early lifeNo good Wikipedia Search Result was foundI should try searching for "Leonardo DiCaprio" instead
Action: Wikipedia
Action Input: Leonardo DiCaprioPage: Leonardo DiCaprio
Summary: Leonardo Wilhelm DiCaprio (; Italian: [diˈkaːprjo]; born November 1I should look for a section on his personal life again
Action: Wikipedia
Action Input: Personal lifePage: Personal life
Summary: Personal life is the course or state of an individual's life, especiallI now know the final answer
Final Answer: Leonardo Wilhelm DiCaprio

> Finished chain.
/Users/harrisonchase/.pyenv/versions/3.10.1/envs/langchain/lib/python3.10/site-packages/wikipedia/wikipedia.py:389: GuessedAtParserWarning: No parser was explicitly specified, so I'm using the best available HTML parser for this system ("lxml"). This usually isn't a problem, but if you run this code on another system, or in a different virtual environment, it may use a different parser and behave differently.

The code that caused this warning is on line 389 of the file /Users/harrisonchase/.pyenv/versions/3.10.1/envs/langchain/lib/python3.10/site-packages/wikipedia/wikipedia.py. To get rid of this warning, pass the additional argument 'features="lxml"' to the BeautifulSoup constructor.

  lis = BeautifulSoup(html).find_all('li')
{'input': "What is Leo DiCaprio's middle name?\n\nAction: Wikipedia",
 'output': 'Leonardo Wilhelm DiCaprio'}